1. Field of the Invention
The present invention relates to an ignition control for a direct fuel injection type internal combustion engine in which gasoline fuel is injected directly into a combustion chamber.
2. Related Art
In a direct fuel injection type internal combustion engine, air-fuel mixture of a very lean air-fuel ratio is burned in combustion chambers for low fuel consumption and low exhaust emission. This lean air-fuel mixture is less ignitable in comparison with an air-fuel mixture of stoichiometric air-fuel ratio by which conventional engines are operated. Therefore, the direct fuel injection type engine is designed to have an improved fuel injection system and improved piston configuration so that fuel-rich region in a generally stratified lean air-fuel mixture is formed near a spark generating area of a spark plug for better ignitability. For instance, as shown in FIG. 8, the fuel-rich mixture formed by air and fuel injected directly from a fuel injector 18 into the combustion chamber is guided toward the spark generating area of a spark plug 17 along a top recess 21 of a piston 20, i.e., from T1 to T3 through T2 in the combustion chamber. The spark plug 17 is activated to generate an electric discharge spark and ignite the fuel-rich mixture first when the fuel-rich mixture arrives at T3. The combustion of the fuel-rich mixture propagates to the fuel-lean mixture existing around the fuel-rich mixture.
However, timing of fuel injection, timing of opening and closing of an intake valve, flow speed of the fuel-rich mixture in the combustion chamber and the like vary from time to time and from engine to engine. Thus, as shown in FIG. 9, time of arrival of the fuel-rich mixture at the spark generating area of the spark plug 17 varies between time t1 and time t2. If the arrival time is later than the time the spark plug 17 generates the electric discharge spark, the fuel-rich mixture will not be ignited resulting in misfire of the stratified air-fuel mixture in the engine. The electric discharge period of the spark plug 17 may be lengthened to ignite even the late-arriving fuel-rich mixture. This will, however, increase the size and weight of the ignition device.